In the production of polyurethanes, it is often desirable to control the activity of the catalyst(s). The effect of controlled catalysis may be realized in improved reactivity profiles, for instance delayed initiation or accelerated cure. Such reaction rate control is of particular importance to the polyurethane molder, where it is important that the polyisocyanate/polyol mixture remain flowable for sufficient time to fill the mold properly, while maintaining or improving demold time. Controlled catalysis can also affect product distributions and significantly impact physical properties of the final polyurethane part.
Latent activity catalysts for initiation delay in polyurethane systems have been reported in the literature. One solution has been the use of ammonium salts of carboxylic acids, alone or in combination with other organometallic catalysts. The disadvantage of such materials is mainly their corrosiveness, but poor polyol masterbatch stability has also been reported in the presence of these acid blocked amine catalysts in U.S. Pat. No. 4,707,501. Bronsted acid blocked amine catalysts are prepared by reaction of a tertiary amine with a glycol borate acid. U.S. Pat. Nos. 3,193,515; 3,127,404 and FR 2,301,554 disclose the use of boric acid in the preparation of a blocked amine catalyst from triethylenediamine and a glycol borate acid. An ammonium salt of a quaternary borate anion results. The advantage of such catalyst composition is delayed activity and/or accelerated cure.
U.S. Pat. No. 5,086,081 describes reduced odor amine-boron compositions prepared from tertiary amine urethane catalysts and boric acid. These compositions are of reduced viscosity, low corrosiveness and impart improved reactivity profiles and physical properties during the production of polyurethane parts. These patents also mention the use of functional equivalents to boric acid (e.g., borate esters such as alkyl, dialkyl and trialkylborates in which the alkoxy groups hydrolyze to hydroxyl functionality in the presence of water).
Quaternary ammonium borates have been used to effect the concurrent trimerization/carbodiimidization of polyisocyanates (U.S. Pat. No. 4,611,013). The borates are prepared from boric acid, alcohols and quaternary ammonium hydroxide and, as such, are not derived from tertiary amines. Other examples of quaternary borate anions as polyurethane catalysts are given in U.S. Pat. Nos. 4,151,334; 3,697,485 and 3,635,848. Borate anions derived from borate esters have also been used for the production of isocyanurate and carbodiimide-isocyanurate foams. These borate anions are prepared by reaction of an alcohol with a borate ester (or boric acid) in the presence of an alkali metal.
The formation of amine-borate complexes has been reported in the literature and appears to depend on both steric and electronic factors. The trialkylborate formed from boric acid and nitrilotriethanol shows no reactivity toward moderate nitrogen nucleophiles, such as diazabicyclo[2.2.2]octane, whereas the related borate formed from 2,2',2"-nitrilotriphenol and boric acid was found to form an adduct with diazabicyclo[2.2.2]octane and other amines (Helv. Chim Acta 1987, 70, 499). Pyridine forms a 1:1 complex with tris-p-chlorophenylborate yet does not react with tris-2,4,6-trichlorophenylborate or 2,6-dimethylphenylborate (J.Chem. Soc. 1956, 3006).
Although acyl borates are known, e.g., boron tris(trifluoroacetate) (Fieser and Fieser 4, 46 and 5, 55), the use of acyl borate-amine complexes as urethane catalysts has not been investigated.
Boron halides are known to form stable complexes with amines. An example would be boron trifluoride-ethylamine (Beil. 4(2), 588). Such materials are well known epoxy curatives and have been used in formulations containing both polyisocyanates and polyepoxides (examples are U.S. Pat. Nos. 4,698,408 and 3,986,991).